1. Field of the Invention
The present invention relates to a light source unit of an optical scanning apparatus used in a writing system of a recording apparatus such as a digital copying machine, a laser printer, a laser facsimile or similar device, and more particularly, to a multi-beam light source unit for use in a multi-beam optical scanning apparatus having a significantly increased recording speed achieved by simultaneously scanning a photosensitive body with a plurality of light beams.
2. Description of Related Art
In an optical scanning apparatus for use in a writing system of a recording apparatus such as a digital copying machine, a laser printer, a laser facsimile or other similar devices, one way of increasing a recording speed is to increase a rotating speed of a rotary polygon mirror used as a light deflector. This solution causes problems with durability of a motor, noise, vibration, and a modulation speed of a semiconductor laser, all of which limit the recording speed.
To solve these problems, there is provided a multi-beam optical scanning apparatus in which a recording speed is increased by scanning a photosensitive body with a plurality of laser beams simultaneously so as to record a plurality of lines simultaneously.
An example of such a multi-beam optical scanning apparatus which is constructed such that light flux from a plurality of semiconductor laser light sources are synthesized by using a beam splitter, and an apparatus with a semiconductor laser array in which a plurality of light emitting sources are arranged in an array, is disclosed in Japanese Unexamined Patent Publication No. 56-42248.
The semiconductor laser array disclosed in JP 56-42248 has a plurality of light emitting sources. However, a common sensor is used for detecting outputs of the plurality of light emitting sources and therefore, it is impossible to perform a feedback of the optical output of each light emitting source in real time unlike a general semiconductor laser. Nonetheless, since the light emitting sources are located close to each other, an optical output of each light emitting source is not constant due to interference between the adjacent light sources, and therefore, accurate and precise control of the total light volume is difficult. In addition, this device is much more expensive than other devices. Thus, the characteristics of the device described above increase the disadvantages as the number of the light emitting sources becomes greater.
On the other hand, the problems as mentioned above do not occur in an apparatus in which a plurality of general-purpose semiconductor lasers are used and light beams from the plurality of semiconductor laser light sources are synthesized. However, the use of semiconductor laser light sources requires improvement of environmental stability and a handling characteristic thereof.
A known multi-beam optical scanning apparatus includes an arrangement in which two light beams from two semiconductor lasers are synthesized and are moved simultaneously by a deflector such as a rotary polygon mirror in order to record adjacent lines.
Recently, it has been necessary to further increase the recording speed and density, and there is provided an apparatus in which a semiconductor laser array is used for the light sources in order to meet this requirement. The semiconductor laser array, however, has control limitations as described above and therefore, it cannot be used in the same manner as a general-purpose semiconductor laser and is inappropriate for high-precision output controls. In addition, since a single collimating lens is used with the plurality of light sources in such an apparatus, a wavelength difference or a difference in a light emitting point position between light sources occurs which causes an error in an image forming position or a diffusion of beams emitted from the collimating lens in directions in which the beams are separated from each other.
On the other hand, in a multi-beam light source unit in which two or more general-purpose semiconductor lasers are used and in which the beams are synthesized with beam splitters or prisms, it is necessary to synthesize the beams in units of a single beam by providing beam splitters or prisms in a plurality of stages. Therefore, the assembly and arrangement of the device becomes extremely complicated. This complicated construction makes it very difficult to perform accurate positioning of the beams or to maintain precise beam spot intervals. In addition, since the complicated construction increases an accumulation of a positioning error, high precision is required for respective parts, and consequently, a correcting unit for detecting a beam position and then correcting the beam position which is deflected with the passage of time is required, which leads to increased cost and assembling difficulty.
In addition, recently, recording apparatuses have been upgraded to include a multi-functioning capability. Also, there is a tendency that the recording apparatuses should provide a function allowing for selection of recording densities according to various types of uses of the recording apparatus. To make the recording densities variable in the recording apparatuses using the multi-beam light source as described, however, it is necessary to make the scanning line intervals in the sub-scanning direction variable in addition to making the modulation speed of a polygon motor or semiconductor lasers variable.